cobalt/sched: add per-policy handler for early parameter checking

We need policy-specific scheduling parameters to be checked early and
unconditionally before applying them in xnsched_setparam(), even if
the current scheduling policy does not change for the target thread
(i.e. sched_declare() is not called).
Signed-off-by: Philippe Gerum <rpm@xenomai.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>